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How to earth came to be
From Wikipedia, the free encyclopedia Jump to navigationJump to search This article is about scientific evidence concerning the history of Earth. For the history of humans, see History of the world. The history of Earth concerns the development of planet Earth from its formation to the present day.12 Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by constant geological change and biological evolution. The geological time scale (GTS), as defined by international convention,3 depicts the large spans of time from the beginning of the Earth to the present, and its divisions chronicle some definitive events of Earth history. (In the graphic: Ga means "billion years ago"; Ma, "million years ago".) Earth formed around 4.54 billion years ago, approximately one-third the age of the universe, by accretion from the solar nebula.456 Volcanic outgassing probably created the primordial atmosphere and then the ocean, but the early atmosphere contained almost no oxygen. Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. While the Earth was in its earliest stage (Early Earth), a giant impact collision with a planet-sized body named Theia is thought to have formed the Moon. Over time, the Earth cooled, causing the formation of a solid crust, and allowing liquid water on the surface. The Hadean eon represents the time before a reliable (fossil) record of life; it began with the formation of the planet and ended 4.0 billion years ago. The following Archean and Proterozoic eons produced the beginnings of life on Earth and its earliest evolution. The succeeding eon is the Phanerozoic, divided into three eras: the Palaeozoic, an era of arthropods, fishes, and the first life on land; the Mesozoic, which spanned the rise, reign, and climactic extinction of the non-avian dinosaurs; and the Cenozoic, which saw the rise of mammals. Recognizable humans emerged at most 2 million years ago, a vanishingly small period on the geological scale. The earliest undisputed evidence of life on Earth dates at least from 3.5 billion years ago,789during the Eoarchean Era, after a geological crust started to solidify following the earlier molten Hadean Eon. There are microbial mat fossils such as stromatolites found in 3.48 billion-year-old sandstone discovered in Western Australia.101112 Other early physical evidence of a biogenic substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in southwestern Greenland13 as well as "remains of biotic life" found in 4.1 billion-year-old rocks in Western Australia.1415 According to one of the researchers, "If life arose relatively quickly on Earth … then it could be common in the universe."14 Photosynthetic organisms appeared between 3.2 and 2.4 billion years ago and began enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose, developed over time, and culminated in the Cambrian Explosion about 541 million years ago. This sudden diversification of life forms produced most of the major phyla known today, and divided the Proterozoic Eon from the Cambrian Period of the Paleozoic Era. It is estimated that 99 percent of all species that ever lived on Earth, over five billion,16 have gone extinct.1718 Estimates on the number of Earth's current species range from 10 million to 14 million,19 of which about 1.2 million are documented, but over 86 percent have not been described.20 However, it was recently claimed that 1 trillion species currently live on Earth, with only one-thousandth of one percent described.21 The Earth's crust has constantly changed since its formation, as has life since its first appearance. Species continue to evolve, taking on new forms, splitting into daughter species, or going extinct in the face of ever-changing physical environments. The process of plate tectonics continues to shape the Earth's continents and oceans and the life they harbor. Human activity is now a dominant force affecting global change, harming the biosphere, the Earth's surface, hydrosphere, and atmosphere with the loss of wild lands, over-exploitation of the oceans, production of greenhouse gases, degradation of the ozone layer, and general degradation of soil, air, and water quality. In the very beginning of earth's history, this planet was a giant, red hot, roiling, boiling sea of molten rock - a magma ocean. The heat had been generated by the repeated high speed collisions of much smaller bodies of space rocks that continually clumped together as they collided to form this planet. As the collisions tapered off the earth began to cool, forming a thin crust on its surface. As the cooling continued, water vapor began to escape and condense in the earth's early atmosphere. Clouds formed and storms raged, raining more and more water down on the primitive earth, cooling the surface further until it was flooded with water, forming the seas. It is theorized that the true age of the earth is about 4.6 billion years old, formed at about the same time as the rest of our solar system. The oldest rocks geologists have been able to find are 3.9 billion years old. Using radiometric dating methods to determine the age of rocks means scientists have to rely on when the rock was initially formed (as in - when its internal minerals first cooled). In the infancy of our home planet the entire earth was molten rock - a magma ocean. Since we can only measure as far back in time as we had solid rock on this planet, we are limited in how we can measure the real age of the earth. Due to the forces of plate tectonics, our planet is also a very dynamic one; new mountains forming, old ones wearing down, volcanoes melting and reshaping new crust. The continual changing and reshaping of the earth's surface that involves the melting down and reconstructing of old rock has pretty much eliminated most of the original rocks that came with earth when it was newly formed. So the age is a theoretical age. When Did Life on Earth Begin? Scientists are still trying to unravel one of the greatest mysteries of earth: When did "life" first appear and how did it happen? It is estimated that the first life forms on earth were primitive, one-celled creatures that appeared about 3 billion years ago. That's pretty much all there was for about the next two billion years. Then suddenly those single celled organisms began to evolve into multicellular organisms. Then an unprecedented profusion of life in incredibly complex forms began to fill the oceans. Some crawled from the seas and took residence on land, perhaps to escape predators in the ocean. A cascading chain of new and increasingly differentiated forms of life appeared all over the planet, only to be virtually annihilated by an unexplained mass extinction. It would be the first of several mass extinctions in Earth's history. Scientists have been looking increasingly to space to explain these mass extinctions that have been happening almost like clockwork since the beginning of "living" time. Perhaps we've been getting periodically belted by more space rocks (ie. asteroids), or the collision of neutron stars happening too close for comfort? Each time a mass extinction occurred, life found a way to come back from the brink. Life has tenaciously clung to this small blue planet for the last three billion years. Scientists are finding new cues as to how life first began on earth in some really interesting places - the deep ocean.